Processes for making cutting instruments

ABSTRACT

THE PRESENT APPLICATION DISCLOSED NOVEL PROCESS FOR PRODUCING CUTTING INSTRUMENTS E.G. RAZOR BLADES, SCALPELS, KNIVES, ETC. FORM MARTENSITIC (CLASS I) STAINLESS (I.E. CONTAINING AT LEAST 10% CHROMIUM) AND SEMI-STAINLESS (I.E. CONTAINING AT LEAST 6% CHROMIUM) STEELS. GENERALLY, THE PROCESSES COMPRISE HEATING AND QUENCHING SUCH STEELS UNDER CONDITIONS WHICH WILL PROVIDE A RETAINED AUSTENITE CONTENT OF AT LEAST 35% AND THEREAFTER HARDENING, THE AREAS OF THE STEEL IN WHICH THE CUTTING EDGES ARE TO BE FORMED BY COLD-WORKING.

United States Patent 3,723,195 PROCESSES FOR MAKING CUTTING INSTRUMENTS Francis E. Flaherty, Canton, and Wyman C. Tupper, Marblehead, Mass., assignors to The Gillette Company, Boston, Mass.

No Drawing. Filed Dec. 3, 1969, Ser. No. 882,345 Int. Cl. C21d 7/02 US. Cl. 14812.4 19 Claims ABSTRACT OF THE DISCLOSURE The present application discloses novel processes for producing cutting instruments e.g. razor blades, scalpels, knives, etc. from martensitic (Class 1) stainless (i.e. containing at least chromium) and semi-stainless (i.e. containing at least 6% chromium) steels. Generally, the processes comprise heating and quenching such steels under conditions which will provide a retained austenite content of at least 35% and thereafter hardening, the areas of the steel in which the cutting edges are to be formed by cold-working.

Up to the present time, cutting instruments, such as razor blades, were formed from martensitic stainless and semi-stainless steels by first hardening such steels by heat treatment to obtain the highest martensite content and lowest austenite content which was achievable e.g. usually less than 30% and thereafter forming the cutting edge in the thus hardened steel by grinding, honing, etc. In the present invention, it has been found that by going contra to the heat-treatment methods heretofore employed, one is able to produce cutting instruments, such as razor blades, scalpels, and cutlery, with superior qualities.

The processes of the present invention comprise first heating and quenching the martensitic stainless and semistainless steel under conditions which will provide a retained austenite content of at least 35% and thereafter hardening, by cold-working, the areas of the steel in which the cutting edges are to be formed. Through the use of the processes disclosed herein, one can produce cutting instruments, with hard edges, and more ductile and corrosion resistant bodies than those heretofore produced from the same martensitic steels using the prior heat treatment hardening methods. Moreover, with martensitic stainless steels i.e. containing at least 10% chromium, one can obtain harder cutting edges than those produced from the same steel using the prior heat treatment methods.

The processes of the present invention have been found to be especially useful in making band or ribbon-like blades for use in reel-type razors, such as disclosed in US. Pat. 3,262,198. Generally, in such reel-type razors, the ribbon-like blade, which is usually only about 1.5 thousandths of an inch thick, is wound around a supply reel from which it is fed along a path to a shaving zone, where portions of its cutting edge are successively exposed, and then on to a takeup reel. In such razors, the ribbon-like blade is subjected to considerable bending which creates stresses therein and makes it susceptible to breaking. This is especially so when the razor is stored in spaces such as medicine cabinets and the like where it may come in contact with corrosive substances and fumes. In order to insure against breaking, it is desirable that at least the body portion of the ribbon-like blade be as ductile and corrosion-resistant as possible. In making such ribbon-like blades using the heat-treatment processes heretofore employed, it was generally necessary to reduce the carbon content below e.g. 0.5% in order to provide the necessary ductility and corrosion resistance. As can be appreciated, any increase in ductility and corrosion resistance which was gained by resorting to such reductions in carbon was accompanied by a loss in edge hardness due Patented Mar. 27, 1973 to the more limited amounts of carbon which were available for hardening. By employing the processes herein disclosed, one is able to produce ribbon-like blades with improved ductility and corrosion resistance without resorting to such reductions in carbon. Moreover, when the ribbon-like blades are made from stainless steels, harder cutting edges can also be obtained.

As can be appreciated, the processes of the present invention are equally useful in producing carving knives, cutlery, scalpels, etc., In end uses such as conventional single and double edge stainless steel razor blades, wherein the ductility and corrosion resistance of the body portions is already considered satisfactory, the processes disclosed herein can be used to produce harder cutting edges.

The martensitic stainless and semi-stainless steels which are used in the processes disclosed herein are generally characterized by the fact that they comprise as essential ingredients: carbon, chromium and iron and are gradually free of austenitizing alloying elements eg nickel, cobalt and manganese in amounts which would prevent the steel from being hardened by heat treatment. They may also contain other alloying agents such as silicon, molybdenum, tungsten, selenium, titanium, columbium, sulfur, phosphorus, aluminum, etc. which, when used in small amounts, well known to metallurigists, modify or improve the properties of the steel. The martensitic steels, which are useful in the processes of the present invention generally comprise: about 6 to about 18% chromium; about 0.4 to about 1.25% carbon and more preferably about 0.5 to 1% carbon, and the balance iron and the small amounts of alloying elements and impurities which may be present. The processes have been found particularly useful with martensitic stainless steels comprising about 10 to about 18% chromium; about 0.4 to about 1.25% carbon and the balance iron and the small amounts of alloying elements and impurities which may be present. Especially useful results have been obtained with stainless steels comprising 12 to 16% chromium and 0.5 to 1% carbon.

As pointed out above, martensitic stainless and semistainless steels are characterized by the fact that they are hardenable by heat treatment. Generally, such heat treatment comprises first heating the steel above the A temperature to convert it to the austentic form, holding it above the A temperature for a suflicient time to dissolve at least a portion of the carbides and thereafter quenching it below the M point (i.e. the temperature at which martensite begins to form) to convert the austenite to martensite. The amount of austenite which will be retained in the quenched steel will principally depend on how high the steel is heated beyond the A point, how long it is held above said point and to what temperature the steel is quenched. Generally, as the steel is progressively heated beyond the A point and held there for progressively longer periods of time, the M point is progressively lowered. It is possible by heating the steel substantially beyond the A point and holding it there for sufficient periods of time to depress the M point to room temperature or below so that upon quenching to room temperature little e.g. 5 to 10% austenite will be converted to martensite. By selecting the temperature beyond the A point, to which the steel is to be heated and held and by controlling the temperature to which the steel is quenched, it is possible to produce steels with varying amounts of retained austenite. In carrying out the processes of the present invention, the steel is generally heated and quenched under conditions which will provide a retained austenite content within the range of about 35 to about 95%. In preferred embodiments of the invention, conditions are chosen which will provide a retained austenite content within the range between about 40 to Generally, the specific temperature to which a steel has to be heated and the period which it has to be held there in order to provide the specified amounts of retained austenite upon quenching will vary depending upon the composition of the steel. Metallurgists are quite familiar with this phenomena and will have little trouble in selecting the appropriate conditions. Generally, with semistainless and stainless sheet stock of about 4 thousandths of an inch thickness, heating the steel to a temperature within the range of about 2000 F. to 2200 F. and holding it at said temperature for a period of at least about 1 min. and thereafter quenching to room temperature will provide the desired retained austenite content. With semistainless and stainless stock of about 1.5 thousandths of an inch thickness the desired retained austenite content may be generally obtained by the holding the stock within the above temperature range for at least about 14 seconds.

For the purposes of the present invention, the retained austenite contents are determined by measuring the saturation magnetic moment per unit volume using a Sucksmith balance as described in Jernkentorets Annaler Arg. 138, 1954; pages 643653.

The cold-working step of the processes of the present invention may be carried out by having the steel at about ambient or atmospheric temperature and thereafter working it by any of the well known methods e.g. rolling, swaging, stomping, pressing, etc. under ambient conditions. Such cold-working may be restricted to the cutting areas or when desired it may be carried out in the body portions of the instrument as well as the cutting edge areas. In the making of cutting instruments with more ductile and corrosion resistant bodies, the cold-working step is generally restricted to the cutting edge areas. In this manner only, the austenite in such areas is converted to martensite and the high retained austenite content e.g. at least 35% is maintained in the body portions of the instrument. It is such high retained austenite, plus the increased amounts of chromium in solution (due to the heat treatment conditions which were initially used to produce the high retained austenite) which is mainly responsible for the increased ductility and corrosion resistance of the cutting instruments. As can be appreciated, the cutting edge areas will vary from cutting instrument to cutting instrument. In most instances, improved ductiling, electro-sharpening, or by a combination of grinding and electro-sharpening.

The cutting edges produced by the processes disclosed herein have good temper-resistance so that they are suitable for being coated with polyfluorocarbon coatings such as the polytetrafluoroethylenes taught in US. Pat. No. 3,071,856 to Irwin W. Fischbein. Such coatings, which are widely used on razor blades usually require sintering at temperatures between about 325 C. to about 400 C. for periods of about 10 to 15 minutes. When stainless steel razor blades, formed by the processes disclosed herein, were coated with such coatings and sintered, it was found that the final hardness of their cutting edges was still appreciably better than the final hardness of cutting edges of control blades which were similarly coated and sintered but were hardened by the prior heat treatment methods.

The following non-limiting examples illustrate the processes of the present invention for preparing razor blades.

EXAMPLE 1 A sheet of martensitic steel comprising about 0.6% carbon, 13.5% chromium and about 0.3% manganese and the balance iron and the usual small amounts of impurities found therein was hot-rolled to a thickness of about 0.0039 inch and cut into strips 0.883 inch wide. The strips were perforated and then run through an elongated furnace which was held at 2100 F. The rate of travel of the strip through the furnace was such that the blade would remain in the furnace for about 163 seconds. The strip upon emerging from the furnace was quenched to room temperature and stress-relieved in boiling water. After being sharpened by grinding and honing, the strip was severed into individual blades. The individual blades were cleaned in trichloroethylene and sprayed with a dispersion of polytetrafiuoroethylene. The coated blades were sintered under a protective atmosphere at 343C. for 10 minutes. The following table gives a comparison of the hardness and temper-resistance of the edges of the blades prepared in this example with blades prepared in a similar manner but hardened according to the prior methods by heating to 2012" F. and quenching to 77 C.

TABLE 1 Percent retained austenite Edge hardness Edge hardness after heat after sharpen- Sintering temperafter sintering treatment and ing (3 kg. load) ature and dura- (3 kg load) Blade quenching equivalent tion of sintering equivalent (1) Blades of Example 1 68.4 848DPHN 343 C. for 10 min- 827DPHN. (2) Heat-hardened blades 14-22 735-752 DPI-IN d0 735-752 DPIIN.

ity will result if at least a substantial portion and preferably a major portion of the body area is not coldworked.

The amount of cold-working which is necessary to harden the cutting edge areas will generally vary with the composition of the steel. Usually, a useful cutting edge can be produced by cold-working the cutting edge areas until the retained austenite content therein is reduced to at least below 30% and preferably to at least below 25% and more preferably below 20%. It has been unexpectedly found that the regular grinding operations which are usually used in sharpening cutting instruments will provide sufficient cold-working to produce a useful cutting edge on semi-stainless steel instruments and to bring about the improved hardness in the cutting edge of stainless instruments. This finding is significant in that it enables one to carry out the process disclosed herein without requiring the purchase of additional cold-working equipment. As can be appreciated, the cutting edge may also be formed by first cold-working at least the areas in which the edge is to be formed by swaging, rolling, pressing, etc. and thereafter forming the cutting edge by grind- As can be observed from Table 1, the blades prepared according to the processes of the present invention have appreciably harder edges both before and after sintering.

EXAMPLE 2 A strip of martensitic steel similar to that used in Example 1 was rolled to a thickness of about 0.0015 inch and cut into ribbon-like blade strips 0.1935 inch wide. A first series of the strips were passed through an elongated furnace at 2150" F. for a period of 14 seconds and quenched to room temperature. The resulting strips had a retained austenite content of 47.3%. A second series of the strips (controls) were passed through the furnace at 2050 F. and quenched to room temperature. The second series of strips had a retained austenite content of 22.8%. Both series of strips were mock sintered at about 343 C. for ten minutes and then subjected to stress corrosion tests. The tests consisted of making, under tension, a 180 4: of an inch diameter) bend in the strips, immersing the bent strips into an aqueous solution containing 50 gms. of sodium sulfate, 0.25 gms. of sodium bisulfate and 0.3 gms. arsenic oxide per liter, passing a current of 0.03 amps per cm. through the strips and observing the strips over a five minute period to record the time at which the strips broke. The results obtained in a series of five tests are set forth in Table 2 below:

Average 1 min., 16 sec As can be noted from Table 2, the strips prepared by the processes disclosed herein have substantially improved stress-corrosion resistance.

EXAMPLE 3 A 0.0039 inch thick strip of martensitic stainless comprising about 1.0% carbon, 13.5% chromium and the balance iron and the usual small amounts of impurities found therein was heated in a furnace at 2050 F. for minutes and quenched to room temperature. The thus heated strip was found to have a retained austenite content in the range between about 75 to 85%. Upon sharpening by conventional methods, the cutting edges had a diamond indent hardness (3 kg. load) of 988 DPHN. When the strips were subjected to mock polytetrafluoroethylene sintering conditions at 635 F. for 1 hour, the edge hardness was about 815 DPHN. When similar strips of steel were heated in a furnace at 1975 F. for about 1 minute and quenched to 50 F. to -95 F., so as to have retained austenite contents of less than the edge hardness after sharpening and being subjected to similar mock sintering conditions, was in the range of about 560 to 600 DPHN.

EXAMPLE 4 A 0.01 inch thick strip of martensitic injector razor blade steel comprising 0.6% carbon, 13.5% chromium, about 0.3% manganese and the balance iron and the usual small amounts of impurities found therein was held in a furnace at 2250 F. for 20 seconds and then quenched to room temperature. The resulting strip which 5 had a retained austenite content of 85% was then coldrolled until a reduction in thickness resulted. The body hardness of the thus rolled strip was 940 DPHN (3 kg. load). When similar strips were hardened, using the prior methods, by holding the strips in a furnace at 2000 F. for 20 seconds and quenching to 77 C., the body hardness averaged about 746 DPHN (3 kg. load).

Having thus described the invention, what is claimed is:

1. A method of making cutting instruments from martensitic stainless and semi-stainless steel, said method comprising heating and quenching said steel under conditions which will provide a retained austenite content between about 35 to 95% and thereafter when the steel is at about ambient temperature hardening the cutting edge areas by cold-working under ambient conditions until the retained austenite content in such areas is at least below 30.

2. A method as defined in claim 1 wherein said martensitic steel comprises about 6 to 18% chromium and about 0.4 to 1.25% carbon.

3. A method as defined in claim 1 wherein said martensitic steel comprises about 10 to 18% chromium and about 0.4 to 1.25% carbon.

4. A method as defined in claim 1 wherein said martensitic steel comprises about 12 to 16% chromium and about 0.5 to 1% carbon.

5. A method as defined in claim 1 wherein said retained austenite content is between about 40 to 90%.

6. A method as defined in claim 3 wherein said retained austenite content is between about 40 to 7. A method as defined in claim 4 wherein said retained austenite content is between about 40 to 90%.

8. A method as defined in claim 1 wherein said coldworking is restricted to the cutting edge areas so that at least a substantial portion of the body areas of said cutting instruments has a retained austenite content of at least 35%.

9. A method as defined in claim 8 wherein said cutting instrument is a razor blade of martensitic steel comprising 10 to 18% chromium and about 0.4 to 1.25% carbon and said heating and quenching is carried out under conditions which will provide a retained austenite between about 40 to 90%.

10. A method as defined in claim 9 wherein said steel comprises between about 12 to 16% chromium and between about 0.5 to 1% carbon.

11. A method as defined in claim 1 wherein said coldworking comprises sharpening by grinding the areas of steel in which the cutting edge is to be formed.

12. A process as defined in claim 1 wherein said steel is stress-relieved in boiling water subsequent to said heating and quenching step and before said cold-working.

13. A razor blade formed from a martensitic steel comprising 6 to 18% chromium and 0.4 to 1.25% carbon, said blade being formed by heating and quenching said steel under conditions which provide a retained austenite content between 35 to and thereafter when the steel is at about ambient temperature hardening the cutting edge areas by cold-working under ambient conditions until the retained austenite content in said cutting edge areas is at least below 30%.

14. A razor blade as defined in claim 13 wherein said steel is heated and quenched under conditions which will provide a retained austenite content between about 40 to 90%. p

15. A razor blade as defined in claim 13 wherein said steel comprises about 10 to 18% chromium and about 0.4 to about 1.25% carbon.

16. A razor blade as defined in claim 13 wherein said steel comprises about 12 to 16% chromium and about 0.5 to 1% carbon.

17. A razor blade as defined in claim 15 wherein said steel is heated and quenched under conditions which will provide a retained austenite content between about 40 to 90%.

18. A razor blade as defined in claim 13 wherein said cold-working is restricted to the cutting edge areas so that at least a substantial portion of the body area of said razor blade has a retained austenite content of at least 35%.

19. A razor blade as defined in claim 17 wherein said cold-working is restricted to the cutting edge areas so that at least a substantial portion of the body area of said razor blade has a retained austenite content of at least 35%.

References Cited UNITED STATES PATENTS 3,425,877 2/1969 Deacon 148--12.4 3,116,180 12/1963 Malzacher 148-12.4 2,717,846 9/1955 Harvey 148-124 3,281,287 10/1966 Edstrorn et al. 14812.4

OTHER REFERENCES Merriman, A. D.; A Dictionary of Metallurgy, 1958, pp. 40 and 41.

WAYLAND W. STALLARD, Primary Examiner 

